This application pertains to the field of antennas and antenna systems and more particularly pertains to antennas for use in wireless communication systems.
Urban and suburban RF environments typically possess multiple reflection, scattering, and diffraction surfaces that can change the polarity of a transmitted signal and also create multiple images of the same signal displaced in time (multipath) at the receiver location. Within these environments, the horizontal and vertical components of the signal will often propagate along different paths, arriving at the receiver decorrelated in time and phase due to the varying coefficients of reflection, transmission, scattering, and diffraction present in the paths actually taken by the signal components. The likely polarization angle between an antenna on a handset used in cellular communication systems and the local earth nadir is approximately 60xc2x0, which may be readily verified by drawing a straight line between the mouth and ear of a typical human head and measuring the angle that the line makes with respect to the vertical. The resulting offset handset antenna propagates nearly equal amplitude horizontal and vertical signals subject to these varying effects of an urban/suburban RF environment. As a mobile phone user moves about in such an environment, the signal amplitude arriving at the antenna on the base station antenna the handset is communicating with will be a summation of random multiple signals in both the vertical and horizontal polarizations.
The summation of the random multiple signals results in a signal having a Rayleigh fading characterized by a rapidly changing amplitude. Because the signal arriving at the base station often has nearly identical average amplitude in the vertical and horizontal polarizations that are decorrelated in time and/or phase, the base station receiver may choose the polarization with the best signal level at a given time (selection diversity) and/or use diversity combining techniques to achieve a significant increase in the signal to noise ratio of the received signal.
Prior art base station antennas that may be used in a selection diversity or diversity combining system often use two separate linearly polarized antennas. This makes for a bulky and unwieldy arrangement because of the space required for each antenna and its associated hardware. U.S. Pat. No. 5,771,024, the contents of which are incorporated by reference, discloses a compact dual polarized split beam or bi-directional array. There is a need in the art, however, for a compact dual polarized boresight array.
The present invention is directed to a dual polarized antenna array for use in wireless communication systems. The antenna array of the present invention may be deployed in relatively small, aesthetically appealing packages and, because the arrays are dual polarized, they may be utilized to provide substantial mitigation of multipath effects.
In one aspect, the present invention is directed to an antenna array comprising a first and a second T-shaped dipole antenna mounted on a ground plane and aligned along mutually parallel axes such that the first and second dipoles transmit and receive a first polarization. A third and a fourth T-shaped dipole antennas are mounted on the ground plane and aligned along mutually parallel axes such that the third and fourth dipoles are aligned to transmit and receive a second polarization, the second polarization being orthogonal to the first polarization. A first equal phase power divider is coupled to the first and second T-shaped dipoles and a second equal phase power divider is coupled to the third and fourth T-shaped dipoles. The first and second T-shaped dipoles are preferably spaced apart broadside to one another approximately a half wavelength of an operating frequency. Similarly, the third and fourth T-shaped dipoles are preferably spaced apart broadside to one another approximately a half wavelength of the operating frequency. Such an array produces a boresight beam with equal elevation and azimuth (E and H plane) beamwidths in both the vertical and horizontal polarizations.
In another innovative aspect of the invention, additional antenna elements are added to produce unequal elevation and azimuth beamwidths. For example, a first and a second T-shaped dipole are mounted along a first axis of a ground plane. A third and a fourth T-shaped dipole are mounted along a second axis of the ground plane wherein the first and second axes are mutually parallel. A fifth, sixth, and a seventh T-shaped dipole are mounted on a third, fourth, and fifth axis of the ground plane, respectively, wherein the third, fourth, and fifth axes are orthogonal to the first and second axes. The fifth, sixth, and seventh T-shaped dipoles are positioned between the first and second axes and the sixth antenna element is positioned between the first and second T-shaped dipoles.
In a preferred embodiment, the first and second T-shaped dipoles are spaced apart a half wavelength of an operating frequency along the first axis. Similarly, the third and fourth T-shaped dipoles are spaced apart a half wavelength of the operating frequency along the second axis that, in turn, is spaced apart a half wavelength from the first axis. Finally, the third, fourth, and fifth axes are spaced apart from one another a half wavelength of the operating frequency. If the first and second axes are positioned to extend in the direction defining vertical polarization, the elevation (E plane) beamwidth of the array is 30xc2x0 whereas the azimuth beamwidth is 65xc2x0 for both the vertically and the horizontally polarized signals. Additional antenna elements can be added along the first and second axes to further narrow the elevation beamwidth.